1. Field of the Invention
The present invention relates to an indentation hardness tester for automatically measuring indentation hardness such as Vickers hardness, Brinell hardness, and the like.
2. Description of the Related Art
Vickers hardness or Brinell hardness is measured to evaluate mechanical characteristics of metal material. A Vickers hardness number is measured by indenting the surface of a test piece with an indenter of a diamond-pyramid shape having face angles of 136 degrees, and by dividing a value of an indenting load by a surface area of a pyramid indentation formed by the indenter. A Brinell number is also measured by dividing a value of an indentation load by a surface area of a spherical indentation formed by an indenter of a steel sphere shape.
A conventional indentation hardness tester is disclosed, for example, in Japanese Examined Patent Publication (Kokoku) No. 63-10379. The indentation hardness tester disclosed in this publication can automatically measure the hardness number, and comprises a testing machine for indenting a surface of a test piece with an indenter under a predetermined load to form indentations, a camera connected to a microscope provided in the testing machine, an A/D conversion part for converting image information obtained in the camera to digital signals in the form of brightness level of, for example, 256 levels, an image processing part for extracting points where the digital image signal from the A/D conversion part abruptly changes, as vertices of the indentation, an indentation size measuring part for calculating the size of the indentation from position data of the vertices of the indentation obtained in the image processing part, and a hardness number calculation part for calculating a hardness number from the size of the indentation calculated in the indentation size measuring part and an indentation load, to output the hardness number to a printer, etc.
The vertices are found by scanning the digital image signal on a measuring axis, in the image processing part. Therefore, if a diagonal line of the indentation does not align with the measuring axis, the indentation size measuring part outputs a wrong value. This situation occurs due to inclination of the indentation caused by microscopic movement in the case where an indented test piece is moved before measurement.
Additionally, if obtained images of the indentation are not clear because of lack of uniformity in brightness of background due to grain boundary, flaw, rust, and the like, the indentation size measuring part likewise outputs a wrong value.
These errors in measurement of hardness number cause a serious problem because the quality characteristics of products can be mistaken.